Simultaneous and extensive removal of the East Asian lithospheric root

Much evidence points to a dramatic thinning of East Asian lithosphere during the Mesozoic, but with little precision on when, or over what time scale. Using geochemical constraints, we examine an extensive compilation of dated volcanic samples from Russia, Mongolia and North China to determine when the lithosphere thinned and how long that process took. Geochemical results suggest that magmatism before 107 Ma derived from metasomatised subcontinental lithospheric mantle (SCLM), whereas after 107 Ma, melt predominantly derived from an asthenospheric source. The switch to an asthenospheric magma source at ~107 Ma occurred in both Mongolia and North China (>1600 km apart), whereas in eastern Russia the switch occurred a little later (~85 Ma). Such a dramatic change to an asthenospheric contribution appears to have taken, from beginning to end, just ~30 Myrs, suggesting this is the duration for lithospheric mantle weakening and removal. Subsequent volcanism, through the Cenozoic in Mongolia and North China does not appear to include any contribution from the removed SCLM, despite melts predominantly deriving from the asthenosphere.


Major-element, trace-element and Sr-Nd-Hf analysis
Samples (TCS 18.1,22.1,25.2,31.1, were analysed for major-and traceelement abundance; and samples (TCS 18.1,22.1,25.2,31.1,33.1 and 56.5) for Sr-Nd-Hf isotope ratios (Extended Data Table 1). The whole-rock samples were crushed in a hardened steel fly press and then powdered in an agate mill; any amygdaloidal material was removed prior to crushing. Major-elements were determined on fusion beads made from pre-ignited powders which were fused with lithium metaborate flux (80% Li metaborate and 20% Li tetraborate) in a ratio of 1:5. Trace-elements were determined on 32 mm diameter pressed powder pellets (10g mixed with 7% PVA solution, as binding agent) to analyse for Rb, Ba, Co,*Th, *U, La, *Ce, *Nd *Sr, Ni, Cr, Pb, Cu, V, Zn, *Nb, La, *Y, and Zr. The analyses were conducted at the University of Leicester by X-ray fluorescence (XRF) spectrometry using a PANalytical ARL 8420 wavelength-dispersive system fitted with a Rh anode X-ray tube and a Philips PW1400 spectrometer with a W anode tube.
Trace, and rare-earth element concentrations (Th, U, Ce, Nd, Sm, Hf, Nb, Sr, Eu, Gd, Tb, Dy, Er, Y, Yb and Lu) were analysed by ICP-MS at the University of Leicester, on a Thermo iCAP-q quadrapole mass spectrometer using a standard HF-HNO3 digestion method from powder. International standards BCR-1 (basalt) and JG-3 (granodiorite), as well as an inhouse standard WS1-b (basalt) and blanks were run with the samples.
Whole-rock samples were analysed for Sr-Nd-Hf isotopes at NERC Isotope Geoscience Laboratory (NIGL), on 150-200 mg of sample. The powder was leached in 5 ml of 6M HCl at 60°C for 2 hours (to remove possible alteration phases) prior to addition of mixed 149 Sm-150 Nd and 176 Lu-177 Hf isotope tracers, and dissolution in HF-HNO3. Primary columns containing Eichrom AG50x8 cation exchange resin were used to separate high-field-strength elements (HFSE), Sr and REE-rich fractions and Hf was separated from the HFSE fraction 1 .
Neodymium was separated from the REE fraction using Eichrom LN-SPEC columns 2 .
Strontium fractions were loaded onto outgassed single Re filaments using a TaO activator solution and analysed in a Thermo-Electron Triton mass spectrometer. Data are normalised to 86 Sr/ 88 Sr = 0.1194. NBS987 standard gave a value of 0.710260 ±0.000006 (9 ppm, 1-sigma, n=16). Sample data are normalised to a preferred standard value of 0.710250.

Isotope Data
Hafnium fractions were dissolved in 1 ml of 2% HNO3 + 0.1M HF, prior to analysis on a Thermo-Electron Neptune mass spectrometer, using a Cetac Aridus II desolvating nebuliser.
Samples were introduced using 0.006 l/min nitrogen, in addition to Ar, to minimise oxide formation. Correction for 176 Yb was made using reverse-mass-bias correction of the 176 Yb/ 173 Yb ratio empirically derived using Hf mass-bias corrected Yb-doped JMC475 The irradiated samples were loaded into an ultra-high vacuum system and a 1059 nm CSI fibre laser was focussed into the sample chamber and was used to step-heat the samples.
After passing through a liquid nitrogen trap, extracted gases were cleaned for 5 minutes using 2 SAES AP-10 getters, one at 450°C and one at room temperature, following which the gases were let into a Nu Noblesse Instruments mass spectrometer for measurement. The mass discrimination value was measured at 295 for 40 Ar/ 36 Ar and system blanks were measured before and after every sample analysis. The system blanks measured before and after every one or two sample analysis were subtracted from the raw sample data. Results were corrected for 37 Ar and 39 Ar decay, and neutron-induced interference reactions. The following correction